A common safety issue with windowed decorative gas fireplaces is the high temperatures that can exist on the window surface. A known method of cooling the exposed surfaces is through the use of inner and outer window panes. Cool air is passed between the two panes to keep the outer pane at a temperature that is safe to the touch. In different fireplaces, that is achieved in different ways.
U.S. Patent Publication No 2005/0139209 to Deng discloses a direct vent type of fireplace that has two separate sources of air. Cooling air is drawn from the room by a fan, is passed between the two panes, and then is released as warm air back into the room. Outside air used for combustion is drawn from the outside into the fireplace, where it is combusted, and then is exhausted outside through an air outlet vent. This fireplace requires at least two air intakes and two outlets. In the Deng type of fireplace, keeping the window panes cool enough to be safe to touch necessarily involves heating the room, which may not be desirable in some cases.
U.S. Pat. No. 5,542,407 to Hawkinson discloses (in Deng's FIG. 2) a system in which room air is still circulated, heated and returned to the room but in a path that does not cool the window panes. Cooling of the panes is by means of outside air that is drawn by natural draft venting through a coaxial vent, passes between the panes from top to bottom where it is then used as combustion air before being vented, again by natural draft venting through the coaxial vent. FIG. 3 of Hawkinson discloses a similar natural draft venting arrangement that does not include a room air heating path. Successful ignition and maintaining combustion in natural draft fireplaces can sometimes be a challenge.
An unvented fireplace is one that exhausts the combustion products directly into the room. U.S. Patent Publication No. 2012/0192854 to Binzer describes an unvented fireplace that has two window panes the exposed window being cooled by the flow of air between two panes. A single air inlet provides room air for both cooling and combustion and the cooling air and the combustion products are exhausted into the room. A restriction in the air pathway at the top of the fireplace promotes the preferential supply of air to the combustion chamber over the interstitial area between the panes.
U.S. Pat. No. 6,848,441 to Bachinski et al. discloses a window pane cooling system that includes an embodiment wherein the outer pane is positioned at an angle with respect to the inner pane. Such an angled pane is said to create a back pressure on the fireplace's blower to increase the flow of cooling air between the two panes. Bachinski et al. claim that the disclosed approach can be used with any type of fireplace, but they do not disclose further details about the supply of combustion air into the combustion chamber.
A prior art type of power vented fireplace that the present invention is intended to directly improve upon is shown in FIG. 1. The prior art fireplace may have one or more viewing sides. Combustion air enters from an outside air inlet and is passed into the combustion chamber. The combustion air flow is said to be “gravity fed”. Air for cooling the viewing panes is drawn by means of cooling air inlet fans from the room and is passed between the two panes. Each side comprises at least one air inlet. The cooling air and combustion air are then mixed in an area above the combustion chamber before being exhausted outside through a single air outlet vent that is sometimes assisted by an exhaust fan. The combustion air is effectively drawn into the combustion air inlet by convection, through the suction created by the remote exhaust fan (if any) and by entrainment from the passing of the cooling air across the top of the combustion chamber. This configuration of power vented fireplace with dual window panes has proven problematic in relation to the balancing of the total air intake from the room air intake fan and outside air inlet with the air being vented out of the exhaust. Depending on the configuration of the mixing area above the combustion chamber and the exhaust fan, if the room air intake fan draws too much air it has a tendency to overpressure the mixing area and prevent air from exiting the combustion chamber thereby choking it and damaging components. When a building is under negative pressure compared to outside air, the tendency to block the combustion chamber outlet is enhanced. The problem may also be exacerbated where a building has fluctuating air pressures, for example restaurants that use exhaust hoods and fans that may be turned on and off at various times. The changes in air pressure in the building upsets the balance between the various fans causing similar problems to the ones noted above.